1. Field of the Invention
The present invention relates to an image reading apparatus for especially reading a color original image and an image reading system using the same.
2. Related Background Art
FIG. 13 is a sectional view showing an example of an image sensor. This sensor is constituted by a sensor array 1 which is prepared by mounting a sensor IC 101 on a sensor board 102, a lens array 2, an LED array 6 having three different LED elements 301, 302, and 303 serving as light sources for illuminating an original, a cover glass 4, and a frame 5 for holding these components in position.
The LED array 6 is constituted by mounting, on the board, a plurality of three different types of LED elements 301, 302, and 303, for example, 20 or more elements on each line to obtain a uniform illuminance distribution of an original upon reading an A4-size original.
The emission peak wavelengths of the three different LED elements 301, 302, and 303 are respectively 430 (nm) corresponding to blue, 570 (nm) corresponding to green, and 660 (nm) corresponding to red.
The lens array 2 is formed by arranging a plurality of columnar lens elements on a line to provide a lens function by setting gradually different refractive indices between the peripheral portion and the central portion.
Also, as the lens array 2, one having a small aperture angle is selected angle since some color originals are adhered with pictures and have three-dimensional patterns on their surface and, hence, a focal length of about 0.5 (mm) is required. Furthermore, a lens array 2 having small chromatic aberrations is selected so that R, G, and B light beams can obtain substantially equal resolutions on the corresponding sensors.
In general, a lens array having an aperture angle of about 12 (deg.) is used, and a conjugate length (TC) as the distance between the original and sensor requires 18.3 (mm).
However, since the above-mentioned conventional image sensor uses a lens array with a small aperture angle, the light amount transmission efficiency is low, and an increase in original reading speed is limited.
In addition, in the lens array with a small aperture angle, distance from the original surface to the sensor so long that size of the image sensor using such the lens array becomes larger.
Furthermore, in the above-mentioned conventional image sensor, since the lens array equally inputs R, G, and B light beams onto the sensor array, a moire phenomenon appears, i.e., a very thin line in the read original is output as a shaggy line in the output image. Such phenomenon is especially conspicuous in a color original output image.